it was his friend and classmate police officer sean collier who ha mortally wounded. >>> the london marathon was run yesterday without incident. before the race started, runners observed a moment of silence in honor of the boston victims. there were more than 34,000 runners. many wore black ribbons to show that they weren't scared away. >>> and now to the small town of west, texas, where over 70 federal and state investigators are trying to figure out what caused a huge fertilizer plant explosion. wednesday's explosion killed at least 14 people, including ten emergency responders. 200 others were injured. and today students return to class for the first time since the blast. manuel bojorquez reports. >> reporter: for the first time news crews were allowed into the disaster area. this apartment building is one of 50 structures that were destroyed. wheelchairs are still lined up on an empty lawn. authorities said they were able to get a closer look at the explosion site. locad the seat of the explosion, which is important because as we conduct our investigation, we'll be workingm temeteinward.